public class Ex4_35{
	public static void main(String[] args){
		double sum = 0;
		for (int i = 1; i <= 624; i++){
			sum += 1.0 / (Math.pow(i, 0.5) + Math.pow(i+1, 0.5));
		}
		System.out.printf("%.3f\n", sum);
	}
}